The present invention relates to the field of thin film solid lubricant materials and structures and to methods of making the same.
In the formation of solid film structures by deposition and growth of the constituent species on a substrate, such as by chemical vapor deposition techniques, sputtering, evaporation and the like, columnar growth structures typically form as an inherent characteristic of the film growth kinetics While the precise scientific reasons for the formation of such columnar growth structures are not fully understood, it is thought that perhaps the tendencies of the kinetics of film growth as they apply to thermodynamically preferred growth structures of the arriving species of deposition as they grow on the substrate tend toward the formation of such columnar structures in the growth process.
At any rate, the formation of such columnar structures in the deposition of such film structures on substrates appears to occur as an inherent characteristic of the growth process In the case of the deposition of thin films of solid MoS.sub.2 using, for example, sputtering deposition techniques, the aforementioned columnar growth structure may typically take the form of the structure illustrated in FIG. 1, which shows a prior art structure of such a MoS.sub.2 film.
Such MoS.sub.2 solid films are used, for example, as solid thin film lubricants which are found to be particularly advantageous for certain applications, such as, for example, high altitude and space applications and other low pressure or vacuum environments. However, the use of such deposited MoS.sub.2 films as solid lubricants has been severely limited by the unavoidable presence of the aforementioned columnar structures in such films.
This same problem of columnar growth occurs in various types of deposited thin films The phenomenon begins to occur in each case at a critical level of thickness in the growth of the thin film. At this critical level of thickness, further growth in thickness is accompanied by increasingly distinct "columns" of growth which develop as adjacent and even touching but structurally separate regions of the film structures. Such columnnar growth structures not only result in the deterioration of the structural integrity of the films in the regions of the columnar structures, but they also result in a reduction in the density and corrosion resistance of the films.
The formation of such columnar growth structures appears to be generic to the process of synthesis of materials by vapor phase deposition, particularly at relatively low substrate temperatures where "metallurgical grains tend to be columnar . . . (with) a growth structure defined by voided boundaries that are also columnar". See J. Thornton, J. Vac. Sci. Technol A4 (6), 3059, (1986). Observation of such columnar structures and theoretical descriptions of their origin are well documented in vapor phase deposition processes. See also, for example, J. A. Thornton, Ann. Rev. Mater. Sci., 7, 239 (1977), and A. Mazor et al., Phys. Rev. Let., 60, 424 (1988). It is believed that similar effects are observed in certain plating processes.
The formation of such columnar structures in solid lubricant films, such as MoS.sub.2 films, severely limits the performance of such films in that the columns break off relatively easily and are subject to removal by wear or adhesion to other parts (such as ball retaining rings) and are thus lost as part of the effective film thickness. This failure mode of the columnar structures adversely affects performance and severely limits the life of the coating.
In the case of MoS.sub.2 films as discussed above, attempts have been made in the prior art to deal with the problem of the formation of such columnar structures in the growth pattern. One such attempt involves the incorporation in the deposition process of additional elements in the film structure which are intended to interfere with and suppress the formation of such columnar structures
A discussion of the problem and the attempts made in the prior art to deal with it is set forth in a paper entitled "A Review of Recent Advances in Solid Film Lubrication" by T. Spalvins, published in J. Vac. Sci. Technol A 5 (2), Mar/Apr 1987. As described in the aforementioned Spalvins paper, the thin film sputter deposition of MoS.sub.2 first forms a ridge-type structure which then transforms into an equiaxed, dense transition zone as the thickness increases before it grows into a columnar fiberlike structural network. Such a prior art structure of a deposited MoS.sub.2 film is shown in FIG. 1 of the drawings of this application.
As pointed out by Spalvins, the equiaxed zone is basically pore free and has a densely packed structure between the equiaxed crystallites and normally does not extend more than about 2,000 angstroms in thickness. The fibers of the columnar structure consist of vertical columns about 2,500 angstroms in diameter which extend perpendicular to the substrate and are separated by open voided boundaries a few hundred angstroms wide. Such deposited films always have a tendency to break within the columnar fiberlike region above the equiaxed zone in a manner which is illustrated in the referenced Spalvins paper.
In the reference Spalvins paper, there is discussed attempts to strengthen the structural integrity of sputtered MoS.sub.2 films "especially in the columnar zone" by introducing gold and nickel dispersions into the sputtered films. However, since the formation of columnar structures appears to be generic, and independent to a great extent of the materials being synthesized, such efforts to stifle or supress formation of the columnar growth structures, or to strengthen the mechanical integrity of such structures, have not been successful.
In addition to the irregular growth structures noted by Spalvins, other structural growth deviations have been observed in the case of MoS.sub.2 films for thicknesses in the range of only a few hundred angstroms, i.e. for thicknesses approaching say 100 or 200 angstroms. The nanostructure of MoS.sub.2 nucleation and growth has been observed by many investigators. Such investigations have shown early growth film morphology consisting of anisotropic (platelike) islands occuring in some cases at thicknesses of just over 100 Angstroms. These structural deviations have also been found to limit the performance and life of such structures as solid lubricants.
Another approach to the improvement of the performance of MoS.sub.2 films, while not addressing the inherent limitations of the columnar structures, has been to employ a two layer coating structure in which a layer of a hard, high modulus material is positioned between the MoS.sub.2 film and the substrate. This hard layer decreases the deflection between the MoS.sub.2 film and the substrate under the film, thereby reducing the contact area between the part and the MoS.sub.2 film and reducing the friction. It has also been reported that such techniques can increase the corrosion resistance of the substrate.
However, such techniques have not addressed directly or provided any means for the effective suppression of the columnar growth structure in deposited thin films, which has remained as a critical problem in such films in the case of deposited films intended for use as solid lubricants.
Certain prior art configurations of MoS.sub.2 thin films have used bilayer or duplex structures in which a single layer of MoS.sub.2 is combined with a layer of another material selected for certain properties For example, one of the problems associated with the use of MoS.sub.2 films is the difficulty of assuring adhesion to certain substrates on which such films are deposited. In order to enhance adhesive properties under such conditions, a thin interlayer is first deposited the substrate and the MoS.sub.2 layer then deposited on the interlayer, the material of which is selected to be compatible with the adhesion of the MoS.sub.2 layer thereto. For example, chemically activated interlayers of rhodium or palladium on steel have been found to promote MoS.sub.2 film adhesion.
In another prior art duplex layer structure, the material of the interlayer which is first deposited on the substrate is selected to provide a hard undersurface for the relatively soft MoS.sub.2 layer which is deposited on top of it, thereby to enhance the hardness of the duplex layer by reason of the firm platform. Hard interlayers of titanium carbide or chromium silicide may, for example, be used for such applications.
The basic problem of the limitation of the useful life of such films has remained, however, limited as explained above by the formation of the columnar structures at film thicknesses above certain critical levels.